1. Field of the Invention
The present invention relates to a light emitting device, and more particularly, to a light emitting device that can minimize reflection or absorption of emitted light, maximize luminous efficiency with the maximum light emitting area, enable uniform current spreading with a small area electrode, and enable mass production at low cost with high reliability and high quality.
2. Description of the Related Art
Light emitting devices include materials that emit light. For example, light emitting diodes (LEDs) are devices that use diodes, to which semiconductors are bonded, convert energy generated by combination of electrons and holes into light, and emit light. The light emitting devices are being widely used as lighting, display devices, and light sources, and development of the light emitting device has been expedited.
In particular, the widespread use of cellular phone keypads, side viewers, and camera flashes, which use GaN-based light emitting diodes that have been actively developed and widely used in recent years, contributed to the active development of general illumination that uses light emitting diodes. Applications of the light emitting diodes, such as backlight units of large TVs, headlights of cars, and general illumination have advanced from small portable products to large products having high power, high efficiency, and high reliability. Therefore, there has been a need for light sources that have characteristics required for the corresponding products.
In general, a semiconductor junction light emitting device has P-type and n-type semiconductor junction structures. In the semiconductor junction structure, light may be emitted by recombination of electrons and holes at a region where the two types of semiconductors are bonded to each other. In order to activate the light emission, an active layer may be formed. The light emitting device having the semiconductor junctions includes a horizontal structure and a vertical structure according to the position of electrodes for semiconductor layers. The vertical structure includes an epi-up structure and a flip-chip structure. As described above, structural characteristics of light emitting devices that are required according to characteristics of individual products are seriously taken into account.
FIGS. 1A and 1B are views illustrating a horizontal light emitting device according to the related art. FIG. 1C is a cross-sectional view illustrating a vertical light emitting device according to the related art. For the convenience of explanation, a description will be made on the assumption that an n-type semiconductor layer is in contact with a substrate, and a p-type semiconductor layer is formed on an active layer.
First, an epi-up light emitting device of a horizontal light emitting device will be described with reference to FIG. 1A.
A light emitting device 1 includes a non-conductive substrate 13, an n-type semiconductor layer 12, an active layer 11, and a p-type semiconductor layer 10. An n-type electrode 15 and a p-type electrode 14 are formed on the n-type semiconductor layer 12 and the p-type semiconductor layer 10, respectively, and, are connected to an external current source (not shown) to apply a voltage.
When a voltage is applied to the light emitting device 1 through the electrodes 14 and 15, electrons move from the n-type semiconductor layer 12, and holes move from the p-type semiconductor layer 10. Light is emitted by recombination of the electrons and the holes. The light emitting device 1 includes the active layer 11, and light is emitted from the active layer 11. In the active layer 11, the light emission of the light emitting device 1 is activated, and light is emitted. In order to make an electrical connection, the n-type electrode and the p-type electrode are located on the n-type semiconductor layer 12 and the p-type semiconductor layer 10, respectively, with the lowest contact resistance values.
The position of the electrode may be changed according to the substrate type. For example, when the substrate 13 is a sapphire substrate that is a non-conductive substrate, the electrode of the n-type semiconductor layer 12 cannot be formed on the non-conductive substrate 13, but on the n-type semiconductor layer 12.
Therefore, referring to FIG. 1A, when the n-type electrode 15 is formed on the n-type semiconductor 12, parts of the p-type semiconductor layer 10 and the active layer 12 that are formed at the upper side are consumed to form an ohmic contact. The formation of the electrode results in a decrease of light emitting area of the light emitting device 1, and thus luminous efficiency also decreases.
In FIG. 1B, a horizontal light emitting device has a structure that increases luminous efficiency. The light emitting device, shown in FIG. 1B, is a flip chip light emitting device 2. A substrate 23 is located at the top. Electrodes 24 and 25 are in contact with electrode contacts 26 and 27, respectively, which are formed on a conductive substrate 28. Light emitted from an active layer 21 is emitted through the substrate 23 regardless of the electrodes 24 and 25. Therefore, the decrease in luminous efficiency that is caused in the light emitting device, shown in FIGS. 1A and 1B, can be prevented.
However, despite the high luminous efficiency of the flip chip light emitting device 2, the n-type electrode and the p-type electrode in the light emitting device 2 need to be disposed in the same plane and bonded. After being bonded, the n-type electrode and the p-type electrode are more likely to be separated from the electrode contacts 26 and 27. For this reason, there is a need for expensive precision processing equipment. This causes an increase in manufacturing costs, a decrease in productivity, a decrease in yield, and a decrease in product reliability.
In order to solve a variety of problems including the above-described problems, a vertical light emitting device that uses a conductive substrate, not the non-conductive substrate, appeared. A light emitting device 3, shown in FIG. 1C, is a vertical light emitting device. When a conductive substrate 33 is used, an n-type electrode 35 may be formed on the substrate 33. The conductive substrate 33 may be formed of a conductive material, for example, Si. In general, it is difficult to form light emitting layers, which include semiconductor layers and an active layer, on the conductive substrate due to lattice-mismatching. Therefore, the light emitting layers grow by using a substrate that allows easy growth of the light emitting layers, and then a conductive substrate is bonded after removing the substrate for growth.
Referring to FIG. 1D, after light emitting layers 30, 31, and 32 are formed, a non-conductive substrate 36 is separated by using a laser. When the laser is irradiated to the non-conductive substrate 36, energy from the laser is absorbed by the semiconductor formed along the boundary between the non-conductive substrate 36 and the p-type semiconductor layer 30. The semiconductor is melted such that the non-conductive substrate 36 is separated from the p-type semiconductor layer 30.
When the non-conductive substrate 36 is removed, the conductive substrate 33 is formed on the n-type semiconductor layer 32, such that the light emitting device 3 has a vertical light emitting structure. When the conductive substrate 33 is used, since a voltage can be applied to the n-type semiconductor layer 32 through the conductive substrate 33, an electrode can be formed on the substrate 33. Therefore, as shown in FIG. 1C, the n-type electrode 35 is formed on the conductive substrate 33, and the p-type electrode 34 is formed on the p-type semiconductor layer 30, such that the light emitting device having the vertical structure can be manufactured.
However, when a high-power light emitting device having a large area is manufactured, an area ratio of the electrode to the substrate needs to be high for current spreading. Therefore, light extraction is limited, light loss is caused by optical absorption, and luminous efficiency decreases. Further, heat, which is generated due to the absorption of the laser energy that is generally used when removing the non-conductive substrate, causes expansion and contraction of the substrate and the semiconductor layers. Stress is applied to each of the layers due to the thermal expansion coefficient and the time difference according to heat transfer. The stress is in proportion to the contact area of the substrate and the semiconductor layers. Therefore, reliability of the large area light emitting device is adversely affected.